The invention pertains generally to the field of slip forming concrete structures and in particular to a machine and method for slip forming the median barrier wall of a highway. These barrier walls are of a generally conventional shape in transverse cross section so as to have a wide base and upwardly and inwardly tapered side surfaces terminating in upright top portions. Since the configuration or contour of the side walls is fixed, such contour remains the same during a slip forming operation. The barrier walls over the linear sections thereof are usually symmetrical relative to a central vertical plane. However, when the slope of the highway varies, as in curves, the barrier wall becomes non-symmetrical relative to such central vertical plane. The general cross sectional shape of the wall is usually dictated by specification requirements providing for sufficient strength to counteract vehicle impact forces and yet having an outer contour of a configuration adapted to minimize the full impact of the vehicle against the wall by providing a rebound action of the vehicle relative to the wall.
Apparatus for forming a wall of this general type is shown in U.S. Pat. No. 3,792,133. The slip form in a first disclosed embodiment in such patent includes a pair of side walls, one of which is fixed. The other side wall is vertically adjustable and has an upper portion integrally formed with a lateraly extended top section or wall. The lower end of the adjustable side wall is movable relative to a fixed plate member, the bottom edge of which is at the level of the lower edge of the fixed side wall.
This disadvantage is partly negatived in a second embodiment of a slip form disclosed in U.S. Pat. No. 3,792,133 by the provision of a pair of selectively adjustable side walls. Thus, when one of the side walls is to be adjusted, the opposite side wall must first be mechanically locked against movement. The slip form of either embodiment is without any means for adjusting the form side walls laterally of the machine frame relative to the form top wall, or for expanding the top wall to vary the thickness of barrier walls having side surfaces of similar configuration. Also, no provision is made for preventing at all times concrete from flowing under the side walls. These objections inherent in the slip form of U.S. Pat. No. 3,792,133 are eliminated by the slip form device and method of the present invention.